The 90MPG is probably the Panamera S e-hybrid. Basically it is designed for a future where internal combustion cars are no longer allowed in crowded city centres, so your execs etc can bomb down the motorway achieving <30mpg and get to the city centre, hit a button and drive on battery power so they can navigate the "zero emissions zone".
I had a 2014 Panamera S e-hybrid for a couple of days...I averaged 25 mpg. I was driving it like a mentalist though.